The purpose of this two-patient feasibility trial is to determine whether normal and/or abnormal hematopoietic progenitor cell populations can be detected in leukapheresis samples obtained from PNH patients and whether such cell populations are detectable using a steady-state leukapheresis, i.e. collection of cells without using techniques designed to enhance release of hematopoietic progenitor cells from bone marrow into peripheral blood.